Condensed cluster phases in reduced oxoniobates: synthesis and studies of Sr4-xNb17O26 (x=0.0(1), 0.3(1)) and Eu4-xNb17O26 (x=0.3(1))

Sr4-xNb17O26 (x=0.0, 0.3) and Eu4-xNb17O26 (x=0.3) were synthesised from SrCO3/Eu2O3 and Nb2O5 in a vacuum furnace at temperatures up to 1600 C (1500 C for Eu) using acetylene soot or Nb as reducing agents. The synthetic studies show that these phases are only formed via an intermediate disordered 'phase' (or phasoid). Sr4-xNb17O26 and Eu4-xNb17O26 are isostructural with Ba4Nb17O26 and crystallise in space group P 4/m (83) with the unit cell parameters for x=0.3: a=12.023(1) A, c=4.1411(4) A and a=12.015(2) A, c=4.135(1) A, Z=1, respectively. The crystal structure can be described as an intergrowth between ANbO3 and NbO. Characteristic building blocks are quadruple chains of corner-sharing Nb6-octahedra. The structures of Sr4-xNb17O26 and Eu4-xNb17O26 were refined using X-ray powder diffraction and neutron diffraction data (only Sr4-xNb17O26). The Rietveld refinements and microanalyses showed x in Eu4-xNb17O26 to be 0.3(1) while in the Sr analogue both x=0.0(1) and 0.3(1) were found. High resolution electron microscopy studies showed that the compounds frequently contained structural defects. The magnetic susceptibility of Eu4-xNb17O26 (x=0.3) shows a Curie-Weiss behaviour, with a magnetic moment in good agreement with the expected μeff=7.9 μB for Eu2+. The Sr analogue is temperature-independent paramagnetic at room temperature. Sr4-xNb17O26 (x=0.3) and Eu4-xNb17O26 (x=0.3) are metallic, with a resistivity increasing with temperature. (orig.)